Authigenic carbonates as tracers of methane-rich fluid emission into the sea floor: fossil examples (Miocene, northern Apennines, Italy) compared to modern analogues

Cold seepage is a worldwide widespread process recognized in different marine settings of both active and passive continental margins. Authigenic carbonates, deriving from anaerobic oxidation of methane, can be used as tracers for ancient hydrocarbon-rich fluid emissions. Seep-carbonates of the northern Apennines mainly formed during the Miocene, commonly located in the inner part of the foredeep, at the deformation front of the accretionary prism. They are concentrated in two different depositional settings: - in pelitic intervals included in Langhian-Serravallian basin-plain turbidites; - in slope hemipelagites (Burdigalian to Messinian) capping turbidites in proximity to the deformational fronts. Although many of these bodies have been studied yet, significant outcrops still have to be investigated. This study will focus on seep carbonates in the Tuscan-Romagna Apennines (Moggiona). Carbonate bodies have lenticular to stratiform morphologies extending along strike and concordant with the attitude of the enclosing marls. Dimensions vary from 10 to 40 m wide and from 3 to 8 m thick. Carbonates are very rich in lucinid-like clams; at the base, numerous conduits filled with coquina debris and breccias are present, as well as dense irregular networks of carbonate-filled veins and extensional fractures. Carbonates are depleted in δ13C ranging from -40.22 to -19.28‰ V-PDB. The comparison between fossil and modern methane-derived carbonates may allow a better definition of the evolution of the seepage system, and relationships with synsedimentary instability (intraformational slumps and extraformational slides) and diapiric processes